The present invention relates to a torque converter with a lock-up clutch.
The torque converter is a power transmission device comprising an impeller shell attached to a front cover, i.e., to a cover of the torque converter connected to an engine-output shaft and is provided with a pump impeller, and a turbine runner, which opposes to the pump impeller and is connected to a main shaft, wherein an engine output is amplified and transmitted to the main shaft that is an output shaft of the torque converter. The torque converter transmits the engine output by utilizing an operating oil as power transmission means, so that a power transmission loss due to a slip of a fluid is inevitable. Therefore, in a region where a rotational difference between the pump impeller and the turbine runner is small, the engine output is directly transmitted to the main shaft by actuating the lock-up clutch.
An ordinary lock-up clutch is provided with a clutch plate connected to the main shaft, thereby directly transmitting the engine output to the main shaft by engaging the clutch plate with the front cover. An operation of the clutch plate is controlled by a differential pressure between the operating oil supplied to a converter chamber which is formed between the clutch plate and the turbine runner, and the operating oil supplied to a clutch chamber formed between the clutch plate and the front cover. That is, in order to release the engagement between the clutch plate and the front cover and free the lock-up clutch, the operating oil a higher pressure than that of an interior of the converter chamber is supplied as a releasing oil pressure to the clutch chamber. When the higher releasing oil pressure than the oil pressure in the converter chamber is provided to the clutch chamber, the clutch plate permanently connected to the front cover is released and the operating oil in the clutch chamber flows into the converter chamber. Since the operating oil is constantly supplied to the converter chamber, the clutch plate is engaged with the front cover due to the oil pressure in the converter chamber if a supply of the operating oil to the clutch chamber is stopped.
As such a lock-up clutch, there is used a multi-plate clutch in which a plurality of the clutch plates are disposed between a clutch drum provided to the front cover and a clutch hub connected to the main shaft (e.g., see Patent Document 1 (Japanese Patent Laid-open No. 2001-163071) and Patent Document 2 (Japanese Patent Laid-open No. 2003-63264)). Each lock-up clutch disclosed in Patent Documents 1 and 2 has a piston for shifting, from a fastening state of engaging the multi-plate clutch and directly connecting the engine output to the main shaft, to a release state of releasing the engagement and for shifting vice versa, and an oil-pressure chamber for fastening is formed between the piston and the front cover. By employing such a multi-plate clutch, since the outer diameter of the clutch plate can be reduced, the lock-up clutch can be easily made compact and therefore a power generating motor constituting a hybrid system can be provided in an empty space obtained by downsizing.
However, if an oil-pressure chamber for an engagement is formed between a piston and a front cover, an oil pressure of a converter chamber is applied to the piston in a clutch-release direction. Therefore, in order to drive the piston against the oil pressure of the converter chamber, which is applied in the release direction, at a time of fastening the clutch, a higher pressure of an operating oil than that of an interior of the converter chamber is required to be supplied to the oil-pressure chamber for fastening. Thus, since the oil pressure for engaging the clutch must be made higher than the oil pressure supplied to the converter chamber, a burden imposed on the oil pump is increased and this is not preferred for improving transmission efficiency. Therefore, a fuel consumption of the engine cannot be reduced. Further, if the diameter of the lock-up piston is made small in order to downsize the lock-up clutch, a pressure receiving area is reduced, so that the oil pressure for an engagement must be made higher. Additionally, if a multi-plate clutch is disposed comparatively away from the main shaft and the diameter of the lock-up piston is enlarged in order to ensure a pressure receiving area, the lock-up clutch is accordingly enlarged in size.